libstagefright: free mMoovBoxBuffer

Add free(mMoovBoxBuffer) in MPEG4Writer::release()

Change-Id: Ic396e64e9e238bdc23cb0b02b7aa341d879d2237
diff --git a/media/libstagefright/MPEG4Writer.cpp b/media/libstagefright/MPEG4Writer.cpp
index e7d3cc2..d53fdab 100644
--- a/media/libstagefright/MPEG4Writer.cpp
+++ b/media/libstagefright/MPEG4Writer.cpp
@@ -810,6 +810,8 @@
     mFd = -1;
     mInitCheck = NO_INIT;
     mStarted = false;
+    free(mMoovBoxBuffer);
+    mMoovBoxBuffer = NULL;
 }
 
 status_t MPEG4Writer::reset() {